The pulse wave arrival time (designated QKd) has been further demonstrated to be useful in evaluating thyroid function under conditions of "diminished thyroid reserve", and in the evaluation of the changes in thyroid hormones which occur during hypocaloric feeding in humans. The QKd has been used to demonstrate that supplementation with oral thyroxin results in improvement at the target organ level, in patients with diminished thyroid reserve as manifested by low or borderline low thyroid function with moderate or minimal TSH elevation. In human subjects undergoing prolonged hypocaloric feeding, normally there is a decrease in serum T4 and T3. Under this set of circumstances, the QKd is prolonged, suggestive of hypothyroidism at the target organ level. Further, oral supplementation with T3, to maintain a euthyroid level of T3, prevents the changes in QKd. In contrast, oral supplementation with T4, which results in supra-normal levels of T4 but slightly decreased levels of T3, fails to provide complete return to normal of the QKd. These findings suggest that QKd is more closely correlated with T3 than with T4 levels, and further suggests that the changes observed in peripheral thyroid hormone levels during hypocaloric feeding is indeed an adaptive mechanism which results in a state of hypometabolism.